1. Field of the Invention
The present invention relates to a method of manufacturing a sintered body from a powdered material.
2. The Prior Art
In the manufacture of tools by the sintering of metal powder bodies, high density and freedom from pores give a high quality product. In the case of cutting tools, the high density gives increased wear resistance and less risks of broken edges. In the case of rollers and the like, the freedom from pores gives increased strength and surface smoothness and this also results in a smoother surface for the product being rolled. Even in the production of electrical resistance bodies of MoSi.sub.2, for instance, there are considerable advantages in having a very high density and freedom from pores. The strength increases and the risk of local over-heating with consequential burning decreases. The advantages of high density and freedom from pores are equally great for cermets of various types.
High density and freedom from pores in sintered products have previously been obtained by enclosing a pressed powder body in a gas-tight, heat-resistance casing of some suitable metal, then evacuating the casing, sealing it and placing it in a furnace wherein the material was sintered under high pressure. Temperatures and pressures of up to 1500.degree. C. and 2000 bars have been used. It is extremely expensive to apply a casing around a pressed body, particularly if it has a complicated shape, to evacuate and seal the casing and finally to remove the casing after the sintering. Especially in the production of small cutting elements the encapsuling is disproportionately expensive. With particularly complicated components, moreover, quite apart from the economic aspects, this method of manufacture simply cannot be used since the casing cannot be removed without damaging the component.
The object of surrounding a powder body to be hot-pressed in a gaseous atmosphere with a gas-tight casing was that the casing should prevent the gaseous pressure medium from coming into contact with the powder body and penetrating into its cavities. Such penetration would result in there being no compaction obtained and hot-pressing under direct influence of a gaseous pressure medium would therefore be pointless. However, it has in recent years proved possible by means of a special method (see German Offenlegungsschrift 2 006 066) to hot-press powder bodies under direct influence of a gaseous pressure medium without enclosing the bodies in a casing. One stipulation for the success of this latter known method, however, is that the bodies consist of a material which during sintering forms a molten phase which closes the pores so that these do not communicate.
The object of the present invention is to provide a process for hot isostatic compacting of powder bodies, in which the bodies do not need to be enclosed in a casing during the compacting process and in which the choice of powder material is relatively wide. This is made possible by the method according to the invention, in which the body of powder material is cold-pressed and then provided with a surface layer of a material having a lower melting point than that of the material of the body or of a material which forms with the material of the body a eutectic which has a lower melting point than that of the material of the body. The body is then placed in a furnace where it is subjected to vacuum and heat, and is thereafter subjected to isostatic hot pressing under the direct influence of an inert gaseous medium. The material forming the outer layer should be at least highly viscous at the sintering temperature of the powder material, and the temperature at which the body is hot-pressed should be sufficient to produce sintering. When using this method the powder material need not include additives with the sole purpose of enabling compacting to take place without the use of a casing, and only such material which will give the final product high quality physical properties need be used. In comparison with a method in which the powder bodies are enclosed in a gas-tight casing, the invention is a considerable simplification. Furthermore, gases in the pores of the powder body can be evacuated more quickly and the evacuation will be more complete since it takes place over the whole surface of the body through the relatively porous surface layer and not only through a thin tube, as is the case when the body in enclosed in a gas-tight casing.